Differential electrochemical mass spectrometry (DEMS) was used to establish the difference in surface acidity in sulfuric acid solution between commercially available carbon black (Vulcan) and Vulcan which had been chemically modified with concentrated nitric acid (VulcanO). The mass signal for the m/z = 2 associated with H2 formation was observed to be influenced by functional groups at the carbon surfaces. Analysis of the mass spectra with the help of the Nernst equation allowed us to quantitatively find the difference in surface pH between the two carbon-based materials in acid media. The elevated concentration of functional groups at the VulcanO surface results in a decrease in the local acidity of the surface under these conditions (0.5 M H2SO4), and consequently the hydrogen evolution reaction (HER) is delayed.

The authors gratefully acknowledge financial support given by Spanish MINECO (ENE2014-52158-C2-2-R and 1-R). GG acknowledges the Viera y Clavijo program (ACIISI & ULL) for financial support.

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Peer reviewed